Studies have been underway to explore new modalities of therapy in reversing drug resistance involving a human colon tumor that expresses the multidrug resistant phenotype. The anti-P-glycoprotein mAb, MRK-16, was compared in vitro and in vivo as intact immunoglobulin, F(ab')2 and Fab for the ability to enhance vincristine sensitivity of the MDR1 retrovirus-infected HT-29 colon adenocarcinoma (HT-29mdr1). The 50% inhibitory concentrations (IC50) of vincristine on parental cells (HT- 29par) and HT-29mdr1 cells were 1.5 and 39.3 ng/ml, respectively. The MRK-16 IgG, F(ab')2, or Fab did not affect vincristine sensitivity of the HT-29par cells. The IC50 of vincristine on HT-29mdr1 cells pretreated with an antigen-saturating concentration of either MRK-16 IgG, F(ab')2, or Fab (67 pmole/ml) was reduced approximately 60% to 15.2, 15.5, and 14.7 ng/ml, respectively. Treatment of mice with vincristine (1 mg/kg) weekly for 3 weeks, beginning 10 days after tumor injection did not increase the median survival time mice bearing the HT- 29mdr1 tumor (39 days versus 35 days). Treatment with MRK-16 IgG (1.67 nmole), 24 h before vincristine therapy increased the survival of HT- 29mdr1 tumor-bearing mice (85 days, p=0.0013). On an equivalent mole basis, the ability of MRK-16 F(ab')2 or Fab to modulate the sensitivity of HT-29mdr1 tumors to vincristine and increase survival was indistinguishable from each other (70 days versus 66 days, respectively), but was statistically different from the intact immunoglobulin (p<0.0031). These data suggest that possible immune- mediated mechanisms potentially attributable to interactions of host effector elements with the Fc portion of the immunoglobulin molecule can be dissociated from the reversal of drug resistance by MRK-16 mAb. Moreover, the data implies that monovalent binding of P-glycoprotein by MRK-16 Fab is sufficient to restore chemosensitivity in multidrug resistant tumors.